Acoustical well logging methods and apparatus



June 25, 1968 I. w. ELLIOTT ET AL 3,390,377

ACOUSTICAL WELL LOGGING METHODS AND APPARATUS Original Filed Sept. 3,1965 4 Sheets-Sheet 1 DISPLAY /20 AND RECORDING APPARATUS 34 1 5 l I IATTENUATION TRAVEL TIME [3 H coMPuTER coMPuTER I 1 I I 1 I 3 I 4 EarthsSurface 4 i CLOCK I PULSE b PuLsE 1 1: l I 1 3'? Low PASS j 9,, m 26 25FILTER I i, ,10 fl 4 ffy 3' 3'2 Low PASS I, k I FILTER I AMPLITUDEDIFFERENTIAL 1 MATcIIIIIIe AMPLIFIER 4 DELAY I L. I, 37 I AMPLITUDEDIFFERENTIAL a MATCHING AMPLIFIER 4'0 3s A Z2 INVENTORS.

JENNINGS W. ELLIOTT 8 DONALD R. GRINE their ATTORNEYS June 25, 1968 J.w. ELLIOTT ET AL 3,390,377

ACOUSTICAL WELL LOGGING METHODS AND APPARATUS Original Filed Sept. 3.1965 4 Sheets-Sheet 2 FIG. 2

BOREHOLE FORMATION FIG. 6

INVENTORS. JENNINGS W ELLIOTT a DONALD R. GRINE June 25, 1968 w, ELLJOTTET AL 3,390,377

ACOUSTICAL WELL LOGGING METHODS AND APPARATUS 4 Sheets-Sheet 3 OriginalFiled Sept. 5. 1965 a 0 S II. W W A8 I. E U 1I|F\ n O N x F M HE R I a I4 ELM m 7 A 8 WW W m hm r TI-PP 1 W J A D A R 0 8 M 21 01 m m z; 7 IA IE9 MA 0 7 N N no WW n u w P... B an 8 m E 8 L 00 I. I T: 7 N $.01 m E m 1om... 3 K 6 m? L D O n s u 0 3 WW uus o l. m? u: w x G S F 71 m U 0 I mT [J 2 R. 11 M6 7 EOV A M 6 U 4 wn s 7 7m S H 6 71MOV THC. w I? 7 P? m mGR I 9 E A mm .2 s mmf/ 11 3 m 6 8 mm E 1 mm P 6 G? 9 7 no" m m}. 3 R E9 AM 60 S a T I. Y m u I SQ- ZIOL F LI 5 o I To OM S 2: o

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ACOUSTICAL WELL LOGGING METHODS AND APPARATUS Original Filed Sept. 5,1965 4 Sheets-Sheet 4 FROM zzno PULSE cnossme eausmwon ozngron I27 I Jcan: l DETECTOR I C I I I 30 VARIABLE R GM" DIFFERENTIAL 2 I AMPLIFIERA'MPLIFIER I m I I 'DETECTOR I I I32 I I GATE I I l TO INVENTORS.D'FFERENT'AL JENNINGS w. ELLIOTT a "5" FIG 5 BYDONALD R. same MEW AMMtheir ATTORNEYS United States Patent Office 3,390,377 Patented June 25,1968 3,390,377 ACOUSTICAL WELL LOGGING METHODS AND APPARATUS Jennings W.Elliott, West Bedding, and Donald R. Grine,

Redding, Conn., assignors to Schlumberger Well Surveying Corporation,Houston, Tex., a corporation of Texas Continuation-impart of applicationSer. No. 484,925,

Sept. 3, 1965. This application June 6, 1967, Ser.

12 Claims. (Cl. 340-18) ABSTRACT OF THE DISCLOSURE An acoustical welllogging system for detecting the shear wave arrivals of an acoustic wavepropagated through earth formations surrounding a borehole bytransmitting acoustic energy from the borehole into the formation,receiving the acoustic energy from the formation at two locations in theborehole spaced different distances from the transmitter, delaying theelectrical signal generated by the receiver located closer to thetransmitter in accordance with the transit time of acoustic energytraveling at formation compressional velocities between the tworeceivers, adjusting the amplitude of the electrical signal generated bythe receiver remote from the transmitter in accordance with theattenuation of acoustic energy traveling at formation compressionalvelocities between the two receivers, and combining the delayedelectrical signal and the adjusted amplitude electrical signal so as tocancel the components of the electrical signals corresponding to thewaves traveling at formation compressional velocities between thetransmitter and the receivers.

This is a continuation of application Ser. No. 484,925, which was filedSept. 3, 1965, and now abandoned.

This invention relates to methods and apparatus for investigating earthformations traversed by a borehole and, more particularly, to improvedmethods and apparatus for making acoustical logs of such formations fromwhich greater information may be obtained than is possible with knownprior art systems.

Apparatus for making such acoustical well logs comprises a logging toolor sonde which is adapted to be passed through the well bore, surfaceequipment for interpreting and recording electrical signals receivedfrom the logging tool, and an interconnecting cable which serves both toconduct electrical signals and power between the tool and the surfaceequipment and also to support the tool during its passage through thebore.

A conventional logging tool may contain, for example, two or moreelectroacoustical transducers, one of which is suitable driven totransmit pulses of acoustic energy through the borehole fluid and intothe surrounding formation. Some of the acoustic energy transmitted tothe formation is refracted back through the drilling mud to the othertransducers, which generate electrical signals in response thereto. Asis well known, the acoustic energy will travel through the formations ascompressional and shear waves and these waves will arrive first at thereceiving transducer or transducers as long as their velocities in theformation are greater than the acoustic velocity in the borehole fluid,which is generally either a water base or an oil based drilling mud. Inthe past, it has been usual to examine the compressional wave arrivals,rather than the slower traveling shear wave components to obtaininformation about formation characteristics. Thus, the velocity ortravel time as well as the amplitude or attenuation of the compressionalwaves are measured in order to obtain an indication of the character ofthe surrounding formation.

While a substantial amount of information can be obtained about aformation from compressional waves travelling therethrough, someimportant formation characteristics cannot be detected with them. Forexample, there is little or no attenuation of a compressional wave by ahorizontal fracture, i.e. a fracture disposed perpendicular to the axisof the borehole, inasmuch as the particle motion of a compressional waveis in the direction of wave travel and compressional waves are readilytransmitted by the fluids normally found in fractures. The transit timeor velocity of a compressional wave (or a shear wave for that matter) isof little use in detecting fractures in most cases because of therelatively low percentage of fracture void to bulk volume. The locationof even very thin fractures is important because oil or gas may beproduced through such fractures, and if necessary these fractures may beopened by conventional means to increase the formation permeability andthe production of hydrocarbons therein.

A shear wave, on the other hand, is highly attenuated by a fluid filledhorizontal fracture, even if the thickness of the fracture is smallcompared to a wavelength so that a reflected wave therefrom isessentially cancelled by interference. This is because the particlemotion of a shear wave is transverse to the direction of travel and asheer wave is essentially not supported by a fluid. The shear waveparticle motion being substantially parallel to the fracture, negligibleenergy from a shear wave is converted to 'be compressional wave at theformation-fluid interface.

Thus, an attenuation log of a shear wave is extremely valuable forlocating horizontal fractures and for determining formationpermeability. In addition, thcre is some evidence that a velocity ortransit time log of a shear wave provides a better measure of porositythan that for a compressional wave. Also, it is useful to have logs ofthe attenuation and velocity of both the shear and compressional wavesfor determining the lithology of the formation.

Unfortunately, shear waves are difficult to detect inasmuch as they aregenerally obscured by prior compressional arrivals. As is well known,the velocity of a shear wave through a formation is considerably lessthan that of a compressional wave. Thus. it has been proposed toseparate the electric signals produced in a receiver by thecompressional and shear waves by making the length of the path travelledby the waves between the transmitter and the receiver a predeterminedvalue which shifts the phase between these waves by degrees. A phase andamplitude detector is then used to separate the two types of signals.However, different ground materials substantially alter the speeds ofpropagation of both types of waves, so that it has not been possible toachieve satisfactory results with this technique.

In accordance with another prior art method, the signals produced by thereceiver are displayed on the screen of a cathode ray tube. An observerestimates the relative attenuation of a first signal corresponding tothe compressional wave and a second signal corresponding to the shearwave. The relative attenuation between the two waves provides anindication of the number and width of fractures extending across thepath between the transmitter and the receiver. It is apparent that thistechnique is time consuming and costly, and has limited application forindustrial use in the field.

Accordingly, it is an object of the present invention to overcome theabove-mentioned difficulties of conventional systems for examiningformations traversed by a well bore.

Another object of the invention is to provide novel methods andapparatus for detecting fractures in a formation through which aborehole extends.

A further object of the invention is to provide improved methods andapparatus for detecting and measuring acoustic shear waves transmittedthrough a formation adjacent a borehole.

These and other objects and advantages of the invention are attained bycombining the acoustic waves received at two locations spaced differentdistances from a single acoustic wave transmitter in the horehole so asto cancel the components of the electrical signals which are generatedby the receivers in response to the waves which travel at compressionalvelocities between the transmitter and the receivers.

The features and advantages of the invention are more fully explained inthe following detailed description thereof when taken in conjunctionwith the accompanying drawings, in which:

FIG. 1 illustrates the well logging tool according to the invention inposition in the well bore in the earth including, in block diagram form,the electronic circuitry therefor;

FIG. 2 is a simplified representation of the acoustic waves formed by asource of sound in a liquid filled borehole;

FIG. 3 is a detailed block diagram of some of the electronic circuitryillustrated in FIG. 1;

FIG. 4 is a schematic circuit diagram of some of the electroniccircuitry illustrated in FIG. 3;

FIG. 5 is a detailed block diagram of some of the electronic circuitryillustrated in FIG. 1; and

FIG. 6 is a series of waveforms useful in explaining the operation ofthe circuit of FIG. 1.

In the embodiment of the invention shown by way of example in thedrawings, a sonde 10 is disposed in a borehole 11 which extends from thesurface through earth formations 12, the borehole being filled with theusual drilling mud 13 which may be of the water base or oil basevariety. The sonde 10 is suspended for movement through the borehole bya multi-conductor armored cable 15 which is wound on a conventionalwinch 17 at the earths surface. The electronic circuitry in the tool 10is powered through the cable 15 by a suitable source of electrical power(not shown) at the earth's surface, and the information for the acousticlogs is transmitted from the sonde through the cable to appropriatedisplay apparatus 20 for making a permanent recording of these logs.Conventional centralizers 21 and 22 are provided adjacent the upper andlower ends of the tool for maintaining the tool properly centered in theborehole.

The sonde 10 mounts four longitudinally spaced electroacousticaltransducers including a transmitter T and three receivers R1, R2 and R3.The transmitter T is preferably a shock excited transducer as disclosedin Patent No. 3,138,219, which issued Jan. 23, 1964. This transmittermay be designed so as to have a dominant frequency of about 20kilocycles per second, for example. The three receivers R1, R2 and R3may include a piezoelectric element of lead zirconate-lead titanateceramic and may be mutually spaced by six inches, for example, while theclosest receiver to the transmitter T may be spaced therefrom by about 3feet.

A clock pulse generator 25 having a repetition rate of 10 cycles persecond drives a conventional pulse generator 26 which in turn excitesthe transmitter T to transmit periodic pulses of acoustic energy throughthe drilling mud 13 as the sonde is moved through the borehole.

FIG. 2 is a simplified illustration of the system of wave frontsexisting near the wall of a liquid-filled borehole shortly afterexcitation of a source of acoustic energy in the hole. The acoustic waveP travels through the drilling mud as a compressional wave at the mudvelocity C. When the acoustic energy strikes the mud-formationinterface, a refracted compressional wave P is generated which travelsthrough the formation at the formation compressional velocity V,,.Although the direction of particle motion due to the compressional waveP is longitudinal, i.e. along the path of propagation, the formationalternately moves into and away from the mud as it is compressed andrarified. This transverse particle motion generates a shear wave in theformation l and also a compressional wave in the mud P P. The latter isa conical mud wave travelling at the mud (compressional) velocity Cwhich is the first arrival at an acoustic receiver in the borehole aslong as the compressional velocity in the formation is higher than inthe mud, and it is this wave that is generally used in conventionaltransit time and attenuation logging. These Waves travel from thetransmitter to the formation as a compressional wave in the mud, arerefracted at the borehole wall, and continue through the formation atthe compressional velocity until they are again refracted to passthrough the mud to the receiver.

When the shear wave velocity V in the formation is greater than theacoustic (compressional) velocity C in the mud, another compressionalconical wave P P is dragged in the borehole by a refracted shear wave PThe formation shear velocity V is always less than the formationcompressional velocity V and so the P P wave is difficult to detect atan acoustic receiver because it is masked by the prior compressional Parrivals and is also interfered with by later compressional arrivalscaused by later transmissions from the acoustic transmitter. Inaddition, interference is caused by the direct mud wave P as well asreverberations thereof in the borehole. These reverberations of the Pwave in the borehole produce a series of P compressional waves in theformation which are not shown in FIG. 2. All of these other F wavestravel at the formation compressional velocity V and are delayed inaccordance with the borehole diameter and the number of reverberations.

Also present are a Stoneley wave S, which travels at a relatively lowvelocity and a pseudo Rayleigh wave R which travels at essentially thesame velocity as the shear wave. Thus the second arrival at an acousticreceiver is actually a compressional wave in response to a combinationof the refracted shear wave P and the pseudo Rayleigh wave R. Forsimplicity the combination of the re fracted shear wave and the pseudoRayleigh wave may be referred to as the shear velocity arrival.

Briefly, the apparatus according to the present inven tion combines theelectrical representations of acoustic signals received by two spacedreceivers so as to cancel the components thereof resulting from therefracted compressional waves thereby leaving the components due to theshear velocity arrival, i.e. the P P waves are cancelled to leave the PP wave. This is accomplished by first delaying the signal received atthe receiver closer to the transmitter by the transit time of therefracted compressional waves between the two receivers, then adjustingthe amplitude of one of the signals to make the amplitudes of the first(compressional) arrivals equal, and finally subtracting one of theresulting signals from the other. It will be recalled that since theformation compressional velocity V exceeds the formation shear velocityV the first portion of each received signal will be the compressionalarrival P P.

Referring now to the block diagram in FIG. 1, the signal received by thereceiver R1 is delayed by a delay circuit 30 in accordance with the timeinterval between the reception of the signals by the receivers R1 andR2, and the output of the delay 3-0 is fed to one input of an amplitudematching circuit 31, the other input of which is supplied by thereceiver R2. The amplitude matching circuit 31 preferably adjusts theamplitude of the first compressional arrival of the signal received bythe receiver R2 to equal that of the signal received by the receiver R1,and the outputs of the amplitude matching circuit are applied to aconventional differential amplifier 32 where the adjusted R2 signal issubtracted from the delayed R1 signal to provide a shear wave arrivalsignal to one of the inputs of conventional attenuation and travel timecomputers 34 and 35, respectively, through a conventional low passfilter 33. The output of the filter 33 is transmitted through the cableto the computers 34 and 35, which are preferably located at the earthssurface along with the display apparatus 20.

Similarly, the signal received by the receiver R2 is delayed by a delaycircuit 3-7 and the amplitude of the R3 signal is adjusted by theamplitude matching circuit 38, the outputs of which are subtracted by aditferential amplifier 40 to produce the P P wave which is received bythe receiver R2. As before, the output of the differential amplifier 40is fed through a low pass filter 41 to the attenuation and travel timecomputers.

Techniques for determining the attenuation and travel time of acousticwaves between a pair of spaced receivers generated from a singletransmitter are well know to the acoustic logging art and so a detaileddiscussion of suitable attenuation and travel time computers 34 and isnot necessary. Similarly, the techniques by which the outputs of suchcomputers are permanently recorded to present logs of attenuation andtravel time as a function of depth in the borehole are well known, andso the display apparatus 20 also need not be discussed in detail.

It should be noted that in the novel arrangement according to thepresent invention, the three receivers R1, R2 and R3 are the equivalentof two receivers, one at R1 and one at R2, each of which provides aninitial segment of an undistorted shear wave arrival, that is, a shearwave arrival that is not masked by prior and later compressional (P P)arrivals. Moreover, the subtraction technique according to the presentinvention cancels not only the compressional P P arrival from the firstP refracted compressional wave, but also the later compressional P Parrivals generated by the reverberations of the P wave in the borehole,since these waves have substan tially the same velocity and attenuationbetween the receivers as that of the first refracted compressionalarrival.

Inasmuch as the compressional wave travel time under conditions in whichshear waves may be detected is from about to 100 microseconds per foot,for a receiver spacing of six inches the delay circuits 30 and 37provide a delay of between 20 and 50 microseconds. FIG. 3 shows a blockdiagram of a suitable form of delay circuit 30, it being understood thatthe delay 37 is identical. The R1 signal is first passed through a 20microsecond conventional delay line 45 so that the remaining variabledelay requirement is from zero to 30 microseconds. The first positivezero crossing of the delayed R1 signal is detected by a conventionalzero crossing detector 46 which triggers a conventional one shot" ormonostable multivir brator 47 which in turn energizes a 100 kilocycleper second conventional pulsed oscillator 48 for the period of timeduring which the R1 signal is of interest. This period of time isselected to insure that enough of the shear wave arrival is detected forproper operation of the attenuation and travel time computers 34 and 35.A conventional flip-flop or bistable multivibrator 50 is turned on byeach pulse from the oscillator 48 thereby opening a conventional gate 51to feed four cycles from a conventional two megacycle oscillator 52 to aconventional two stage counter including a pair of flip-flops 53 and 54,the output of the flip-flop 54 turning off the Hipflop 50 and therebyde-energizing the gate 51.

Three conventional AND circuits 57, 58 and 59 are connected to theflip-flops 53 and 54 to decode the state of the counter and providethree consecutive 0.5 microsecond pulses for driving three sample andhold circuits 61, 62 and 63. Each sample and hold circuit, whenenergized, transfers the voltage applied to the input thereof to thecorresponding one of the capacitors 65, 66 and 67 connected to itsoutput. The sequence of the sampling pulses is such that the voltagestored on the capacitor 66 is first transferred to the capacitor 65,then the voltage on the capacitor 67 is transferred to the capacitor 66,and finally the level of the delayed R1 signal from the delay line 45 istransferred from the input of the sample and hold circuit 63 to thecapacitor 67. The three sample and hold circuits 61, 62 and 63 aretherefore each pulsed once for each 100 kilocycle per second clock pulsefrom the oscillator 48. The three sample and hold circuits with thethree storage capacitors thus comprise a dynamic delay line 68 in whichthe samples of the R1 signal are shifted through one stage every tenmicroseconds.

When the R2 signal arrives, the first sample of the R1 signal will bestored either on the capacitor 67 (corresponding to a 2030 microseconddifference between the R1 and R2 signals) on the capacitor 66(corresponding to a 30-40 microsecond difference), or on the capacitor(corresponding to a 40-50 microsecond difference).

In order to know where the first sample of the R1 signal is stored whenthe R2 signal arrives, a counter including a pair of flip-flops 70 and71 counts the number of 100 kilocycle per second clock pulses occurringduring the interval between the R1 and R2 signal arrivals. When the R1signal is detected, the one shot multivibrator 47 turns on a fiip-fiop72 which opens a gate 73 which feeds the 100 kc. clock pulses to thislast named counter. The first positive zero crossing of the R2 signal isdetected by a conventional zero crossing detector 76 which turns on aone shot multivibrator 77 for the length of time during which the R2signal is of interest. The multivibrator 77 turns otf the flip-flop 72at the first positive zero crossing of the R2 signal, thus closing thegate 73 and stopping the counter including the flip-flops 70 and 71 atthe count which indicates which of the capacitors 65, 66 and 67 isstoring the first sample of the R1 signal. This counter is decoded bythree conventional AND circuits 80, 81 and 82. The outputs of these ANDcircuits are connected to a corresponding one of the inputs of threegates 84, 85 and 86, respectively, the other inputs of which areconnected to the capacitors 65, 66 and 67 respectively. In this way thesample of the R1 signal is read out of the proper one of the three stagedynamic delay line 68.

The multivibrator 77 triggered by the R2 signal also energizes a secondkilocycle per second pulsed oscillator 90, each output pulse of whichopens at gate 92 through a flip-flop 93 to feed two cycles from the twomegacycle clock oscillator 52 to a flip-flop 94, inasmuch as theflip-flop 94 turns off the flip-flop 93. The output of the fiip-fiop 94is thus a 0.5 microsecond pulse which drives a sample and hold circuit96 to transfer a sample of the R2 signal at the input thereof to astorage capacitor 97. At the same time that the R2 signal is so sampled,the gated output of the R1 dynamic delay line, i.e. the output of theappropriate one of the gates 84, 85 and 86, is transferred by a sampleand hold circuit 98 to a storage capacitor 99, so that the R1 and R2signals appear on the storage capacitors 97 and 99 as two series ofsamples which are exactly in phase.

The clock signals provided by the two megacycle oscillator 52 to thesample and hold circuits 96 and 98 are of opposite phase to that usedfor stepping the samples of the R1 signal between stages of the dynamicdelay line 68, so that it is not possible to read out from this delayline while the line is active.

FIG. 4 illustrates the schematic circuit diagram of the sample and holdcircuit 63 of FIG. 3, it being understood that the other sample and holdcircuits are identical. An electrical signal which is to be sampled isapplied to an input terminal 101 which is coupled through a resistor 102to ground and is connected directly to the bases of a pair oftransistors 103 and 104, which, in turn, are coupled in parallel betweena terminal 105, adapted to be connected to a source of positive voltage(not shown), and a terminal 106, adapted to be connected to a source ofnegative voltage (not shown).

The collector of the transistor 103 is connected directly to theterminal 106, and the emitter thereof is coupled through a resistor 107to the terminal 105 and through a diode 108 to a terminal 109.Similarly, the collector of the transistor 104 is connected directly tothe terminal 105, while the emitter thereof is coupled through aresistor 111 to the terminal 106 and through a diode 112 to a terminal113. The polarity of the diode 112 is such that it is conductive whenthe potential of the terminal 113 exceeds that of the emitter of thetransistor 104, while the polarity of the diode 108 is such that it isconductive when the potential of the emitter of the transistor 103exceeds that of the terminal 109.

The emitters of the transistors 103 and 104 are also connected to thebases of a pair of transistors 115 and 116, respectively, which areconnected back-to-back in series between the terminals 105 and 106.Thus, the collectors of the transistors 115 and 116 are connecteddirectly to the terminals 105 and 106, while the emitters of thesetransistors are coupled together through a pair of series-connectedresistors 117 and 118. The output of the sample and hold circuit appearsat a terminal 120 which is connected to the junction of the resistors117 and 118 and which may be connected to the storage capacitor 67 ofthe dynamic delay line 68, for example.

In order to sample a signal at the terminal 101, a pair of oppositepolarity sampling pulses are applied to the terminals 109 and 113. Thusa positive pulse from the AND circuit 59, for example, is applied to theterminal 109 and to a conventional inverter circuit (not shown) whichsimultaneously supplies a negative pulse to the terminal 113. Thesampling pulses disable the diodes 108 and 112 which otherwise clamp theemitters of the transistors 103 and 104 to prevent the transmission ofthe input signal at the terminal 101 therethrough. Thus, during theduration of the sampling pulses, both positive and negative inputsignals may be transmitted through two signal paths, defined by thetransistors 103 and 115 and the transistors 104 and 118, into thestorage capacitor 67.

This sample and hold circuit thus provides bilateral charging of thestorage capacitor 67 so that the level on this capacitor can be changedrapidly in either direction. Also, each of these two signal paths,including the transistors 103 and 115 and the transistors 104 and 116,has two junctions of opposite polarity in series so that changes inbase-emitter voltage drop tend to cancel. Furthermore, the leakagecurrents in the output transistors 115 and 116 are of opposite sign andtend to cancel each other.

FIG. illustrates the amplitude matching circuit 31, it being understoodthat the amplitude matching circuit 38 is identical. The sampled R2signal appearing on the storage capacitor 97 of FIG. 3 is fed to aconventional variable gain amplifier 125, the gain of which iscontrolled by circuitry now to be explained so that the amplitude of thecompressional arrival at the output thereof equals that of the delayedR1 signal appearing on the storage capacitor 99. The output of thevariable gain amplifier 125 is fed directly to the differentialamplifier 32, and also through a gate 126 to a conventional square lawdetector 127. The gate 126 is enabled by a conventional pulse generator128, which is triggered by the zerocrossing detector 76 (see FIG. 3),when the R2 signal arrives. The duration of the output pulse from thegating pulse generator 128 is such that three or four excursions of theR2 compressional arrival, for example, are fed to the detector 127,which thus measures the energy of the portion of the R2 signal fedthereto. The output of the detector 127 drives one input of aconventional differential amplifier 130.

Similarly, the same portion of the R1 compressional arrival is fed to asquare law detector 131 through a gate 132 which is also enabled by thepulse generator 128. The output of the detector 131 drives the otherinput of the differential amplifier 130, the output difference signal ofwhich is fed to the gain-controlling electrode of the variable gainamplifier 125. The amplifier 125 is thus automatically controlled by thedifferential amplifier 130 so that the amplitudes of the R1 and R2compressional P P arrivals fed to the differential amplifier 32 areequal.

The differential amplifier 32 then subtracts the adjusted amplitude R2signal from the delayed R1 signal to cancel the compressional P P wavesthereby leaving the shear wave arrival which is fed to the attenuationand travel time computers as discussed above.

The low-pass filters 33 and 41 smooth or remove the stepped nature ofthe waveforms fed thereto, which are caused by the sampling accomplishedin the delay circuits 30 and 37. Inasmuch as the sampling rate iskilocycles per second, these low-pass filters preferably cut off sharplyat 50 kilocycles per second. Negligible information is lost from theshear wave signals by removing the components therefrom havingfrequencies higher than 50 kilocycles per second, and it is well knownfrom information theory that any signal can be sampled at a rate onlyslightly higher than twice the highest frequency component containedtherein without losing any information contained in the original signal.

FIG. 6 shows waveforms which illustrate the manner in which the shearwave arrival may be recovered from the acoustic waves received at twospaced locations from a single transmitter in a borehole by cancellingthe compressional P P waves in accordance with the present invention.The waveforms 6A, 6B and 6C illustrate typical acoustic waves receivedby the receivers R1, R2 and R3, respectively. These waveforms are soaligned that the delay of the first arrivals between the receivers hasbeen removed. Furthermore, the amplitudes of the first (compressional)arrivals are the same. The waveform 6D was obtained from the subtractionof waveform 68 from GA, and it is apparent that the compressionalarrival has been eliminated leaving only the desired shear wave arrival.Similarly, waveform 6E was obtained by subtracting waveform 6C from 6B.It may be observed that the shear wave arrival in waveform 6E(corresponding to the receiver R2) is delayed from that in waveform 6D(corresponding to the receiver R1), inasmuch as the spread between thecompressional and shear Wave arrivals increases in proportion to thedistance from the acoustic transmitter.

While the fundamental novel features of the invention have been shownand described, it will be understood that various substitutions, changesand modifications in the form and details of the apparatus illustratedand its manner of operation may be made by those skilled in the artwithout departing from the spirit of the invention. Thus it is apparentthat the novel subtraction technique may be used with acoustic waves ofany desired frequency, appropriate adju tment being made in the spacingbetween the transmitter T and the receivers R1, R2 and R3. Furthermore,the R1, R2 and R3 signals may be transmitted through the cable 15 aftersuitable amplification, the other circuitry discussed above beinglocated at the earths surface. Interchannel crosstalk may be minimizedby conventional multiplexing techniques. For example, conventional timedelay networks may be employed to transmit the three signals up thecable sequentially, suitable gating circuits rejecting the laterarriving direct mud waves P as necessary. All such variations andmodifications therefore, are included within the intended scope of theinvention as defined by the following claims.

We claim:

1. Apparatus for detecting the shear wave arrival in an acoustic wavereceived in a borehole extending through an earth formation, comprisingmeans for transmitting acoustic energy from the borehole into theformation, first receiving means at a first location in the borehole forgenerating a first electrical signal in response to the acoustic energyreceived from the formation, second receiving means at a second locationin the borehole for generating a second electrical signal in response tothe acoustic energy received from the formation, the second receivingmeans being located at a greater distance from the transmitting meansthan is the first receiving means, means for delaying the firstelectrical signal in accordance with the transit time of acoustic energytraveling at formation compressional velocities between the first andsecond locations, means for adjusting the amplitude of the secondelectrical signal in accordance with the attenuation of acoustic energytraveling at formation compressional velocities between the first andsecond locations, and means for subtracting one from the other of thedelayed first electrical signal and the adjusted second electricalsignal.

2. Apparatus for examining a formation through which a borehole extends,comprising transmitting means for emitting acoustic energy in a boreholeopposite a formation, first transducer means in first spaced relation tothe transmitting means for generating a first electrical signal inresponse to the acoustic energy received from the formation, secondtransducer means in second, farther spaced relation to the transmittingmeans for generating a second electrical signal in response to theacoustic energy received from the formation, means for delaying thefirst electrical signal in accordance with the transit time of acousticenergy traveling in a compressional mode over a spacing corresponding tothat between the first and second transducer means, means for adjustingthe amplitude of the second electrical signal in accordance with theattenuation of acoustic energy traveling in a compressional mode over aspacing corresponding to that between the first and second transducermeans, and means responsive to the delayed first electrical signal andthe adjusted second electrical signal to produce a further signal whichvaries as a function of the difference between said first and secondsignals.

3. Apparatus for examining a formation through which a borehole extends,comprising means for transmitting acoustic energy from the borehole intothe formation, first transducer means in first spaced relation to thetransmitting means for generating a first electrical signal in responseto the acoustic energy received from the formation, second transducermeans in second spaced relation to the transmitting means for generatinga second electrical signal in response to the acoustic energy receivedfrom the formation, means for delaying the first electrical signal inaccordance with the transit time of acoustic energy traveling atformation compressional velocities between the first and secondtransducer means, means for adjusting the amplitude of the secondelectrical signal in accordance with the attenuation of acoustic energytraveling at formation compressional velocities between the first andsecond transducer means, means for subtracting one from the other of thedelayed first electrical signal and the adjusted second electricalsignal to produce a first subtracted signal.

4. Apparatus according to claim 3, including means responsive to thefirst subtracted signal for determining the attenuation suffered by theacoustic energy between the transmitting means and one of the first andsecond transducer means.

5. Apparatus according to claim 3, including means responsive to thefirst subtracted signal for determining the transmission time of theacoustic energy between the transmitting means and one of the first andsecond transducer means.

6. Apparatus according to claim 3, wherein the delaying means includesmeans for detecting the arrival of the first electrical signal, meansresponsive to the first signal detecting means for generating at leastone first sampling pulse, means responsive to the first sampling pulsegenerating means for sampling a portion of the first electrical signal,means for storing the sampled portion of the first electrical signal,means for detecting the arrival of the second electrical signal, meansresponsive to the second signal detecting means for generating at leastone second sampling pulse, and means responsive to the second samplingpulse generating means for simultaneously transferring a portion of thesecond electrical signal to the amplitude adjusting means andtransferring the stored sampled portion of the first electrical signalto the subtracting means.

7. Apparatus for examining a formation through which a borehole extends,comprising means for transmitting acoustic energy from the borehole intothe formation, first transducer means in first spaced relation to thetransmitting means for generating a first electrical signal in responseto the acoustic energy received from the formation, second transducermeans in second spaced relation to the transmitting means for generatinga second electrical signal in response to the acoustic energy receivedfrom the formation, means for delaying the first electrical signal inaccordance with the transit time of acoustic energy traveling atformation compressional velocities between the first and secondtransducer means, means for adjusting the amplitude of the secondelectrical signal in accordance with the attenuation of acoustic energytraveling at formation compressional velocities between the first andsecond transducer means, means for subtracting one from the other of thedelayed first electrical signal and the adjusted second electricalsignal to produce a first subtracted signal, third transducer means inthird spaced relation to the transmitting means for generating a thirdelectrical signal in response to the acoustic energy re ceived from theformation, means for delaying the second electrical signal in accordancewith the transit time of acoustic energy traveling at formationcompressional velocities between the second and third transducer means.means for adjusting the amplitude of the third electrical signal inaccordance with the attenuation of acoustic energy traveling atformation compressional velocities between the second and thirdtransducer means, and means for subtracting one from the other of thedelayed second electrical signal and the adjusted third electricalsignal to produce a second subtracted signal.

8. Apparatus according to claim 7, wherein the subtracting meanssubtracts the adjusted second electrical signal from the delayed firstelectrical signal and subtracts the adjusted third electrical signalfrom the delayed second electrical signal, and including meansresponsive to the first and second subtracted signals for determiningthe attenuation suffered by the acoustic energy between the first andsecond transducer means.

9. Apparatus according to claim 8, including means responsive to thefirst and second subtracted signals for determining the transmissiontime of the acoustic energy between the first and second transducermeans.

10. Apparatus according to claim 9, including means for recording theattenuation and transmission time determined at various depths in theborehole.

11. Apparatus for delaying a first electrical signal so as to be inphase with a later arriving second electrical signal, comprising meansfor detecting the arrival of the first electrical signal, meansresponsive to the first signal detecting means for generating at leastone first sampling pulse, means responsive to the first sampling pulsegenerating means for sampling a portion of the first electrical signal,means for storing the sampled portion of the first electrical signal,means for detecting the arrival of the second electrical signal, meansresponsive to the second signal detecting means for generating at leastone second sampling pulse, and means responsive to the second samplingpulse generating means for simultaneously transferring a portion of thesecond electrical signal to a first output terminal and transferring thestored sampled portion of the first electrical signal to a second outputterminal.

12. Apparatus for delaying a first electrical signal so as to be inphase with a later arriving second electrical signal, comprising meansfor detecting the arrival of the first electrical signal, meansresponsive to the first signal detecting means for generating at leastone first sampling pulse, means responsive to the first sampling pulsegenerating means for sampling a portion of the first electrical signal,means for storing the sampled portion of the first electrical signal,means for detecting the arrival of the second electrical signal, meansresponsive to the second signal detecting means for generating at leastone second sampling pulse, and means responsive to the second samplingpulse generating means for simultaneously transferring a portion of thesecond electrical signal to a first output terminal and transferring thestored sampled portion of the first electrical signal to a second outputterminal, wherein the storing means includes a plurality of cascadedstorage elements and at least one means for transferring the signalstored on each storage element to the next storage element, eachtransferring means being coupled between a different pair of adjacentstorage elements, and including means responsive to each first samplingpulse for generating a plurality of successive transfer pulses, andmeans for coupling each of the transfer pulses to a different one of thetransferring means, each transferring means transferring the signalstored on a different storage element to the next storage element whenactivated by a transfer pulse.

References Cited UNITED STATES PATENTS 2,691,422 10/1954 Summers et al181-.5 2,943,694 7/ 1960 Goodman 18l-.5 3,013,211 12/1961 Garabedian328-109 3,143,666 8/1964 Aaronson 328155 X 3,177,467 4/1965 Brokaw340'--18 3,213,375 10/1965 St. John 328-72 X 3,252,099 5/1966 Dodd328151 X 3,252,131 5/1966 Vogel 181-.5 3,259,880 7/1966 Zemanek 340183,310,751 3/1967 Atzenbeck 328-151 X 3,333,238 7/1967 Caldwell 181.5

OTHER REFERENCES Pickett, Acoustic Character Logs in FormationEvaluation, Journal of Petroleum Technology, June 1963, pp. 659-667.

RODNEY D. BENNETT, Primary Examiner.

BENJAMIN A. BORCHELT, Examiner.

R. M. SKOLNIK, D. C. KAUFMAN,

Assistant Examiners.

